1. Technical Field
The present invention relates to a fixing unit used in an image forming apparatus for an electrophotographic manner, and more particularly to a fixing unit having a rotatable belt member.
2. Related Art
In image forming apparatuses such as copiers and printers employing an electrophotographic manner, an image is formed as follows. First, for example, the surface of a photosensitive member (photosensitive drum) formed in a drum shape is uniformly charged by a charging unit. The charged photosensitive drum is scanned and exposed by light controlled in accordance with image information and an electrostatic latent image is formed the surface thereof. Subsequently, the electrostatic latent image on the photosensitive drum is made into a visible image (toner image) by a developing unit and the toner image is transported to a transfer unit, where the toner image is electrostatically transferred onto a sheet of recording paper. The toner image transferred onto the sheet of recording paper is subjected to a fixing process, thereby completing the toner image.
The fixing unit used in such an image forming apparatus has, for example, a configuration that a fixing roll in which a heat-resistant elastic layer and a detachment layer are stacked on the surface of a cylinder-shaped core bar having a heating source (heater) disposed therein and a pressurization roll in which a heat-resistant elastic layer and a detachment layer made of a heat-resistant resin film or a heat-resistant rubber film are stacked on a core bar come in close contact with each other. A toner image is fixed onto a recording sheet by allowing the recording sheet having supported a non-fixed toner image to pass through a contact area (nip portion) between the fixing roll and the pressurization roll and performing a heating and pressurizing process to the non-fixed toner image. Such a fixing unit is referred to as a two-roll type fixing unit and has been widely used in general.
In recent years, increase in productivity and color has been rapidly advanced in image forming apparatuses and apparatuses having a double-sided printing mechanism have been spread more and more. Accordingly, a fixing unit which can cope with increase in process speed has been required more and more.
However, when the increase in process speed is accomplished by the use of the two-roll type fixing unit, it is difficult to sufficiently perform a fixing process to plural recording sheets continuously supplied for a short time. That is, in the two-roll type fixing unit, since the core bar constituting the fixing roll or the elastic layer made of a silicon rubber coated on the core bar serve as heat-resistant bodies, it is difficult to instantaneously and sufficiently supply a heat amount, which corresponds to the heat amount released from the surface of the fixing roll to the recording sheets, from a heater disposed in the fixing roll.
Therefore, a surface temperature of the fixing roll gradually decreases during the continuous feed of sheets to gradually deteriorate the fixing performance. In addition, at the time of initiating the image forming apparatus, a so-called “temperature dropping phenomenon” that the surface temperature of the fixing roll temporarily droops can occur easily. Specifically, when thick sheets of paper having a great heat capacity are used as the recording sheets, the heat amount released from the surface of the fixing roll increases. Accordingly, the deterioration in fixing performance or the droop in temperature increases, thereby easily causing deterioration in image quality due to the fixing failure.
Accordingly, a technology for solving the above-mentioned problems in a case that the two-roll type fixing unit is used and embodying a fixing unit coping with the increase in speed of an image forming apparatus, there is known a technology relating to a fixing unit in which a heating member is constructed to suspend a film-shaped belt member (fixing belt) on plural tension rolls. That is, there is known a technology of fixing a toner image by previously heating the fixing belt with a heater disposed inside the tension rolls before the fixing belt enters the nip portion and heating the recording sheet and the toner image through the heated fixing belt in the nip portion.
In the fixing unit employing such a belt member, since the heat capacity of the belt member is small, it is easy to recover the belt member to a predetermined fixing temperature for a short time even when heat is released to the recording sheet during the fixing process. Therefore, such a configuration of the fixing unit is very suitable for accomplishing the increase in speed of the image forming apparatus.
In the fixing unit employing the fixing belt, a toner image is supported on the surface of a recording sheet. Accordingly, when the toner image is fused by the heat from the fixing belt, the toner image serves as adhesive and an adhesive force acts between the recording sheet and the fixing belt. Therefore, it is necessary to provide a mechanism for stripping the recording sheet from the surface of the fixing belt. Specifically, when the increase in speed of the image forming apparatus is required and the detachment failure occurs in the fixing unit to cause a jam of paper, the number of successive recording sheets to be damaged increase due to the influence of the jam of paper. Accordingly, it is necessary to stably detach the recording sheet having passed through the nip portion at a high speed from the fixing belt.
As the mechanism for stripping the recording sheet from the surface of the fixing belt, a configuration that a stripping claw is disposed at the downstream side of the nip portion to abut the fixing belt is employed. Further, in a configuration that the fixing belt stretched on the fixing roll and the heating roll and the pressurization roll are disposed in close contact with each other, a fixation member for setting the curvature of the fixing belt great at an exit portion of the nip portion is provided at the inside of the fixing belt and at the position corresponding to the exit portion (the most downstream portion) so as to detach the recording sheet by the use of variation in curvature of the fixing belt.
However, in a fixing unit using a fixing belt, when a stripping claw is used as the mechanism for stripping the recording sheet from a surface of the fixing belt, it is necessary to dispose the stripping claw in contact with the fixing belt so as to stably detach a recording sheet from the fixing belt. As a result, the surface of the fixing belt can be easily worn out by the stripping claw. When a worn mark is generated on the surface of the fixing belt, a fixing stain corresponding to the worn mark can be generated on a fixed image, thereby deteriorating image quality. In addition, toners offset onto the worn mark can be gradually deposited, thereby generating contaminations on the fixed image. Furthermore, when the wear of the fixing belt is advanced, the thin fixing belt may be finally destroyed, thereby damaging the function of the fixing unit.
When a fixation member for making the curvature of the fixing belt great is disposed at an exit of a nip portion as a mechanism for stripping the recording sheet from the surface of the fixing belt, the fixing belt comes in close contact with a pressurization roll by only a tension of the fixing belt at an intermediate nip region between an entrance of the nip portion where the fixing roll and the pressurization roll come in close contact with each other and the exit in which the fixation member is disposed. Accordingly, the nip pressure at the intermediate nip region is relatively low. When the recording sheet or the toner is heated in a low nip-pressure region, moisture in the recording sheet can be vaporized into steam or air in the toner can be thermally expanded, thereby generating air gaps (bubbles) between the fixing belt and the pressurization roll. When such air gaps are generated, a non-fixed toner can be easily disturbed because the air gaps are floated in a state that the toner on the recording sheet positioned in the nip portion is not completely fixed. As a result, image defects such as stains may be easily generated in a fixed image.
In the fixing unit using the fixing belt, when the stripping claw is used as the mechanism for stripping the recording sheet from the surface of the fixing belt, it is necessary to dispose the stripping claw in contact with the fixing belt so as to stably detach the recording sheet from the fixing belt. As a result, when the stripping claw is used, the surface of the fixing belt can be easily worn out by the stripping claw. When a worn mark is generated on the surface of the fixing belt, a fixing stain corresponding to the worn mark on the surface of the fixing belt can be generated on the fixed image, thereby deteriorating image quality. In addition, toners offset onto the worn mark can be gradually deposited, thereby generating contaminations on the fixed image. Furthermore, when the wear of the surface of the fixing belt is advanced, the thin fixing belt may be finally destroyed, thereby damaging the function of the fixing unit.
When the fixation member for making the curvature of the fixing belt great is disposed at the exit of the nip portion as the mechanism for stripping the recording sheet from the surface of the fixing belt, the fixing belt comes in close contact with the pressurization roll by only the tension of the fixing belt at an intermediate nip region between an entrance of the nip portion where the fixing roll and the pressurization roll comes in close contact with each other and the exit in which the fixation member is disposed. Accordingly, the nip pressure at the intermediate nip region is relatively low. When the recording sheet or the toner is heated in the low nip-pressure region, moisture in the recording sheet can be vaporized into steam or air in the toner can be thermally expanded, thereby generating air gaps (bubbles) between the fixing belt and the pressurization roll. When such air gaps are generated, the non-fixed toner can be easily disturbed because the bubbles are floated in the state that the toner on the recording sheet positioned in the nip portion is not completely fixed. As a result, image defects such as stains may be easily generated in a fixed image, thereby causing the deterioration in image quality.